neutrino• The W then decays to an electron and a neutrino.• Massless electron neutrinos playing at being heavy tau neutrinos might help solve either or both.• A sensible theorist's approach is to hope that the fatneutrino goes away.• Thus oscillatingneutrinos must have mass; but massive neutrinos need not oscillate.• An array a squarekilometre in size should see neutrinosources if there are any, Halzen says.• Up until now there has been no need to believe that neutrinos have any mass at all.• And doing this thrilled me to my teeny-tiniest neutrino.